The first successful, economical, and widely accepted means of performing digital type control was by means of the classic relay. The early system design engineers essentially grew up with relay coils and relay contacts; often priding themselves in their ability to go through a maze of contacts and relay coils to understand the control strategy. More times than not, the only person that could understand the control scheme was the person that designed the system.
With the advent of solid state, a family of building blocks was soon available for the system designer. Again, a designer would configure a control system this time using AND's OR's, NOT's, NAND's, etc. Some of the relay type design people could change over to the "new" way of configuring or representing a control system; while others could not.
One of the major disadvantages of these two approaches is that both are hard wired schemes, meaning field changes are very difficult to make and require physical removal and replacement of wiring.
The programmable controller was then developed to primarily overcome the field change problem. The programmable controller incorporated both of the above schemes. Solid state was used to implement the logic because of its reliability and space saving. The relay symbol was retained to represent the control scheme. Once again a design engineer would be responsible for the design of a control system using relay coils and contacts. The configured system would then be used to program the programmable controller. Various methods are used for programming, depending on the particular manufacturer. Factory or field changes are easily made simply by changing the program, in fact a complete new control scheme can be implemented in very short order.
Another means used for process control is the computer. Again the control scheme is configured using relay symbology or flow diagrams. These diagrams must then be converted to some coding which the computer will accept. This coding is usually done by a programmer. The design engineer usually cannot implement a control system directly on the computer because he does not have the proper background or the understanding of computers to do the programming. On the other hand, the programmer is able to program the computer but he does not have the background to understand a particular process to configure a control scheme.
The development of a process control system usually involves a number of individuals and sometimes two or more companies (the customer, consulting firm, and the manufacturer of the control system). In the earlier days, the means of communicating a particular system configuration between individuals and companies was done by means of relay symbology and later with the use of AND's, OR's, NOT's, etc. Through the years, control systems in general have become more complex and thus communication is even a greater problem. Only skilled individuals could really understand a control scheme.
Shortly after the advent of the computer, a new form of logic representation evolved, this was the flow diagram, sometimes called logic diagram. The flow diagram does not relate to any particular implementation such as relays, solid state, computer, etc. Its function is to describe the control scheme of a process. This is done by means of various sizes and shapes of "boxes" in which a desired action or question is written. So a person no longer had to be skilled in relay diagrams or other diagrams to design or understand a control system.
This method of representing logic control vastly improved the communication of a desired control scheme between individuals and companies. A larger number of people could now participate in the design or approval of a particular system. As for the manufacturer, he still had and has a problem. He must now convert the flow diagrams to some other diagram depending on the method of implementation. (Relay diagram for relays, AND, OR, NOT, etc. for solid state, relay diagrams or Boolean expressions for the programmable controller, etc.). This conversion is both time consuming and costly.
It is therefore an object of the present invention to provide a programmable controller which has all the flexibility of existing programmable controllers and the capability of being programmed from a number of widely accepted process representation.
It is another object of the present invention to avoid and eliminate the costly and time consuming use of ladder diagrams, Boolean equations, etc. and utilize flow diagrams as a process representation.
It is another object of the present invention to provide a programmable controller utilizing a human language format which may be derived from an inspection and analysis of the flow diagram of the particular process under consideration.